1. Field of the Invention
The present invention relates to a subscriber terminal, particularly for use in a passive optical communication network (PON) in which a plurality of subscriber terminals are connected for communication to a central-office unit. The invention also relates to a method for adjusting the intensity of an optical signal in such a subscriber terminal.
2. Description of the Background Art
As a typical example of a communication system between a central-office unit and a plurality of subscriber terminals, a passive optical network (PON) is known. The PON system includes a single central-office unit, a plurality of subscriber terminals and an optical splitter, which interconnects the central-office unit and the subscriber terminals with optical fibers. Applications where the PON system is collaborated with the code division multiplexing (CDM) scheme can enjoy various advantages inherent to the CDM scheme.
Telecommunications network systems utilizing the CDM scheme use a code common to both transmitter and receiver sides, thus accomplishing high security for communication. Additionally, the network system utilizing the CDM scheme allows transmission data from plural sources to be multiplexed in a single time slot. The use of the CDM scheme thus accomplishes a larger capacity of data communications with communication resources, such as time slots, saved as referred to in U.S. Pat. No. 7,630,642 B1 to Tamai et al., for example.
Optical fibers for use in the PON system bring an attenuation rate of about 0.5 dB/km. In such a case, a subscriber terminal residing at a distance from the optical splitter further than another subscriber terminal by 10 km, for example, may receive an optical signal transmitted by the central-office unit weaker in optical intensity than the other subscriber terminal by about 5 dB. If the subscriber terminals have the tolerable range thereof, i.e. dynamic or receivable range, equal to about 5 dB, some of the subscriber terminals distancing themselves from the optical splitter further than others by 10 km or more may receive optical signals out of the dynamic ranges, thus failing to properly receive the optical signals.
Particularly, when the PON system is powered on to perform a presence check for the subscriber terminals by the central-office unit, the subscriber terminals may receive a too strong intensity of optical signals transmitted by the central-office unit, thus failing to properly receive the signals. As a result, the central-office unit may not able to check the presence of the subscriber terminals.